Electric motor control circuit

ABSTRACT

A control circuit for an electric motor is proposed, whose rate of rotation and rotational direction depends on a control signal (Ue) at the input terminal (10). The rotational direction is given by means of a determined control signal level being exceeded or fallen short of. The rate of rotation is controlled by means of a pulse width modulation of the control signal (Ue). There is a nonlinear relationship between the change in the pulse duty factor of the pulse width modulation and the control signal. The mean adjusting speed of an actuating motor accordingly adapts to the control deviation signal of an actuating control.

PRIOR ART

The invention is based on a control circuit for an electric motor whoserotational direction and rate of rotation are controlled by a controlsignal whereby the rotational direction is controlled upon exceeding orfalling below a predetermined level of the control signal, and the rateof rotation is controlled by the pulse-width modulation of the controlsignal. Such a circuit arrangement, which generates a bidirectionalpulse signal to two outputs as a function of the amplitude and thepolarity of an analog input signal, is known from the companypublication: "Voltage Regulators with Applications", pages 111 to 118,Astronic, Winzererstrasse 47d, 8000 Munich 40, 1982. This circuitarrangement is offered in the form of an integrated circuit. It is usedin servo systems which are controlled by electric motors and forcontrolling the rate of rotation of electric motors. The integratedswitching circuit contains a delta connected voltage generator, acontrol circuit for the delta voltage generator and two comparators. Thepulse width modulation is effected by adding the delta voltage and theanalog control signal and subsequently comparing the resulting signalwith an upper and lower threshold with the aid of the two comparators.There is always a linear relationship between the analog control signaland the pulse width.

ADVANTAGES OF THE INVENTION

In contrast, the control circuit, according to the invention, with thecharacterizing features of claim 1 has the advantage that a pulse widthmodulation is effected with a simpler circuit, wherein a nonlinearrelationship can be predetermined between the analog control signal andthe pulse width.

The control circuit contains few constructional elements and thereforemakes possible a construction in series production at low cost. It isparticularly suitable for adapting the mean adjusting rate of anactuating motor to the level of the analog control signal, which, forexample, can be a command variable or the control deviation signal of anactuating control. Within the range of small deviations between theactual and desired values of the rate of rotation of the motor, thenonlinearity offers the advantage of a more rapid elimination of thedeviation without the risk of overshooting the desired value.

Other details and advantageous developments of the control circuit,according to the invention, follow from the subclaims in connection withthe following description.

DRAWING

FIG. 1 shows a wiring diagram of the control circuit, according to theinvention;

FIGS. 2 to 4 show the dependence of the control voltage Ue on aninternal signal voltage Ui.

DESCRIPTION OF THE EMBODIMENT EXAMPLE

In the wiring diagram (FIG. 1) of the control circuit, a control signalUe arrives at an input terminal 10 and then at a junction 12 via aresistor 11. The non-inverting input of a first operational amplifier 13and the inverting input of a second operational amplifier 14 areconnected at the junction 12. The signal of a rectangular wave generator15 is connected to the junction 12 via a capacitor 16. Moreover, thejunction 12 is connected with a circuit ground 18 via a resistor 17. Theinverting input of the first operational amplifier 13 is connected witha supply voltage of the control circuit at the connection 20 via aresistor 19 and the non-inverting input of the second operationalamplifier 14 is connected with the circuit ground 18 via a resistor 21.A resistor 22 is located between the inverting input of the firstoperational amplifier 13 and the non-inverting input of the secondoperational amplifier 14. An electric motor 25 is connected between theoutput 23 of the first operational amplifier 13 and the output 24 of thesecond operational amplifier 14. The positive pole of a first voltagesource 26 is connected to the first operational amplifier 13 and itsnegative pole is connected to the circuit ground 18. A second voltagesource 27 has its negative pole connected to the second operationalamplifier 14 and its positive pole connected to the circuit ground 18.

The control circuit, according to the invention, works in the followingmanner: At the junction 12 the attenuation control signal Ue at theinput terminal 10 is added to a pulse-shaped voltage. The weakening ofthe control signal Ue is determined by a voltage divider circuit to thetwo resistors 11, 17. The pulse-shaped voltage is obtained afterdifferentiation of the output voltage of the rectangular wave generator15 with the aid of the capacitor 16 and the resistor 17. Thepulse-shaped voltage consists of positive and negative needle pulseswith a subsequent exponentially damped waveform, wherein the needlepulses occur in the rising and falling sides of the rectangular wavesignal. The exponentially damped waveform depends on the values of thecapacitor 16 and the parallel equivalent circuit of the two resistors11, 17. This internal signal voltage Ui at the junction 12 is alsoconnected to the non-inverting input of the first operational amplifier13 and to the inverting input of the second operational amplifier 14,which are both wired as comparators. The switching threshold of the twocomparators is derived by means of a voltage divider circuit includingthe three resistors 19, 22, 21, which are connected in series, thevoltage divider circuit being connected between the supply voltage ofthe control circuit at the connection 20 and the circuit ground 18. Theupper threshold voltage Uo occurs between the two resistors 19, 22 andis applied to the inverting input of the first operational amplifier 13.The lower threshold voltage Uu occurs between the resistors 21, 22 andis applied to the non-inverting input of the second operationalamplifier 14.

The output of the two operational amplifiers 13, 14 is constructed ineach instance as a power semi-bridge circuit. The electric motor 25 isconnected via these power semi-bridges either to one of the two voltagesources 26, 27, which have a voltage of +Ub or -Ub, or to none at all.The exact control of the electric motor 25 is explained with the aid ofFIGS. 2 to 4:

FIG. 2 shows the case in which the electric motor 25 obtains no power.If the internal signal voltage Ui lies within the voltage rangedetermined by the upper switching threshold Uo and the lower switchingthreshold Uu, then the outputs 23, 24 of the two operational amplifiers13, 14 are connected to the circuit ground 18.

FIG. 3 shows the example of a positive displacement of the controlsignal Ue at the input terminal 10. The positive "needles" of theinternal signal voltage Ui exceed the upper switching threshold Uo ofthe first operational amplifier 13, which is wired as a comparator. Aslong as the upper switching threshold Uo of the first operationalamplifier 13 is exceeded, its output 23 is connected with the positivepole of the voltage source 26. In addition, the output 24 of the secondoperational amplifier 13 is connected to the circuit ground and thevoltage Um for a rotational direction, which voltage is approximatelyequal to the voltage +Ub of the first voltage source 26, is accordinglyconnected to the electric motor 25.

FIG. 4 shows the case of a negative displacement of the control signalat the input terminal 10. If the internal signal voltage Ui falls belowthe lower switching threshold Uu of the second operational amplifier 14,which is wired as a comparator, then its output 24 is connected with thenegative pole of the second voltage source 27. The output 23 of thefirst operational amplifier 13 is connected to the circuit ground 18.The voltage Um for the opposite running direction, which voltage isapproximately equal to the voltage -Ub of the second voltage source 27,is connected to the electric motor 25.

The nonlinear relationship between the control signal and the pulsewidth occurs to the following manner: if the upper or the lowerswitching threshold, as shown in FIG. 3 of 4, is only slightly exceededor fallen short of, respectively, a comparatively short power supplypulse is generated in a positive or negative direction. On the otherhand, if the switching thresholds are exceeded or fallen short of to amore considerable extent this leads to a comparatively long positive ornegative power supply pulse. Small deviations of the actual value of therate of rotation of the electric motor 25 from the desired value areaccordingly controlled in a "smooth" manner to a certain extent. Largerdeviations, on the other hand, lead to a more rapid follow-up.

The nonlinearity depends on the exponential drop of the pulse-shapedvoltage. It is determined by means of the value of the capacitor 16, thevalue of the resistor 17 and the value of the resistor 11, which for analternating voltage is likewise connected to the circuit ground 18. Theamplitude of the positive and negative needle pulses dependsadditionally on the amplitude of the rectangular wave voltage as well asits frequency. The frequency of the rectangular wave voltage is selectedin such a way that a continuous current flow is adjusted at theinductive load which in addition to a work load is represented by theelectric motor 25.

We claim:
 1. A circuit for controlling rotational direction and rate ofrotation of an electric motor, comprising a source of a first drivevoltage connected to said motor via a first comparator, a source of asecond drive voltage of opposite polarity connected to said motor via asecond comparator, each of said comparators having two inputs, one inputof said first comparator being connected to a first reference voltage todetermine an upper threshold, and one input of said second comparatorbeing connected to a second reference voltage to determine a lowerthreshold, a rectangular wave generator connected via a differentiatorto the second input of each of said comparators to apply theretodifferentiated pulses of opposite polarity, and a source of a controlsignal connected to the second input of each of said comparators tosuperpose said differentiated pulses whereby said first drive voltageactivates said motor in a rotational direction when the resultingsuperposed signal exceeds said upper threshold, said second drivevoltage activates said motor in the opposite rotational direction whensaid superposed signal falls below said lower threshold, and the rate ofrotation being controlled by the pulse width modulation of said controlsignal.
 2. A circuit according to claim 1, wherein said upper thresholdis determined by said first reference voltage at the inverting input ofsaid first comparator which comprises a first operational amplifier, andsaid lower threshold is determined by said second reference voltage atthe non-inverting input of said second comparator which comprises asecond operational amplifier.
 3. A circuit according to claim 2, whereinsaid first and second reference voltages are determined by means of avoltage divider arrangement.
 4. A circuit according to claim 3, whereinsaid electric motor (25) is connected to the output of said first andsecond operational amplifiers which contain a power semi-bridge outputstage in each instance.
 5. A circuit as defined in claim 1 wherein saiddifferentiator is an RC member.
 6. A circuit as defined in claim 1wherein said differentiated pulses of opposite polarity are needlepulses with an exponentially damped slope to produce a non-linear pulsewidth modulation of said control signal.